Thermal spraying techniques have been used to apply durable coatings to a wide range of substrates. These substrates can be metallic, composite, or ceramic. A wide variety of metallic alloys and ceramic compositions have been used as coating materials to provide improved hardness, corrosion resistance, wear resistance, oxidation resistance or other surface modifications to the substrate in combination with the appropriate surface texture. One desirable group of coating materials are the MCrA1Y family including nickel, cobalt, or other alloys. MCrAlY materials are generally ally comprised of a base metal M (including Ni,Co,Fe, and mixtures of these elements), Cr, Al and Y. Modifications of these coatings have included additions of other materials such as Si, Ta, Hf, W, Re and others, to enhance the resistance of such materials to high temperature oxidation and corrosion.
It has long been known that the presence of oxygen during the plasma or HVOF operation will often promote unwanted oxidation of the coating making the coating unacceptable. Because the ambient atmosphere contains approximately 21% O.sub.2, i.e. 210,000 parts per million (ppm) of O.sub.2, the need readily arises to remove the spray process from ambient atmosphere. As such, the spray operation has often been contained in an enclosed chamber to control the ambient atmosphere therein. One standard technique has been to create near vacuum conditions, under 175 torr, in an enclosed chamber with less than 5 ppm of O.sub.2. These standard vacuum processes are often referred to as LPPS as trademarked by Electroplasma, Inc. (from low pressure plasma spray) or VPS for vacuum plasma spraying. One such process and chamber is disclosed in U.S. Pat. No. 4,853,250 issued to Boulos et al. on Aug. 1, 1989.
These LPPS methods also require sealed chambers with access doors that can be tightly sealed and mechanical pumps to create the vacuum within the chamber that adds to the expense of the chamber. Furthermore, the nature of the operation is that the doors must be opened and closed before and after each plasma coating operation which constrains this type of coating operation to a less efficient and more time consuming batch type operation.
It has been found that oxidation during the LPPS or VPS process occurs during two steps of the operation. Firstly, oxidation may occur as the powder particles are "air born" as it is being applied to the substrate. Secondly, oxidation may occur after application onto the substrate before it solidifies. While these LPPS and VPS chambers successfully removed most oxygen from the chamber, and thus are very effective in eliminating the oxidation during the particle "air born" phase of the application, the LPPS high temperatures and thus relatively slow cooling rates on the substrate allow most of the oxidation to occur during the cool down phase of the coating.
U.S. Pat. No. 4,587,135 issued to Diener et al. on May 6, 1986 discloses a plasma method using a closed chamber at elevated pressures set at 1-40 torr above atmospheric pressure. The pressure is controlled by a gas relief valve in communication with the interior of the chamber which allows escapement of the gas only when it exceed a predetermined pressure over ambient to minimize backwash or turbulence that might allow the introduction of oxygen back into the chamber.
Previous attempts to provide an adequately dense, adherent MCrA1Y coating and elimination of oxidation by using an open chamber have failed. Analysis of the failures indicate that the failure emanates from the high oxygen atmosphere and turbulence created by the thermal spray process. In an open chamber the introduction of a thermal spray nozzle and high volumes of inert or low oxygen gases might be viewed as a method of preventing coating oxidation and reaction. However, the high turbulence and resulting presence of significant concentrations of oxygen create unsatisfactory results.
The present invention relates to the fact that it has been found that most of the oxidation occurs after application of the coating onto the substrate. Lowering the temperatures of the substrate and thus shortening of the cool down period can reduce the oxidation even if higher oxygen content exists in the ambient plasma spray.
What is needed is a chamber free from seals and pressure relief valves in which the oxygen content can be adequately controlled by the input of displacing gases at controlled rates into the chamber that is continuously open through a restricted opening to the surrounding ambient atmosphere rather than by the evacuation of the oxygen from a sealed and enclosed chamber. It is thus possible to provide for a tunable chamber that can be easily adjusted to control the desired amount of the oxygen present during the spray coating operation.